tiny refactor of invoice setting logic
diff --git a/erpnext/accounts/doctype/subscriptions/subscriptions.py b/erpnext/accounts/doctype/subscriptions/subscriptions.py
index bbf2a69..0a2eea3 100644
--- a/erpnext/accounts/doctype/subscriptions/subscriptions.py
+++ b/erpnext/accounts/doctype/subscriptions/subscriptions.py
@@ -267,17 +267,16 @@
 		)
 
 		# Discounts
-		if self.apply_additional_discount:
-			invoice.apply_discount_on = self.apply_additional_discount
-		else:
-			invoice.apply_discount_on = 'Grand Total'
-
 		if self.additional_discount_percentage:
 			invoice.additional_discount_percentage = self.additional_discount_percentage
 
 		if self.additional_discount_amount:
 			invoice.additional_discount_amount = self.additional_discount_amount
 
+		if (self.additional_discount_percentage or self.additional_discount_amount) \
+			and not self.apply_additional_discount:
+			self.apply_additional_discount = 'Grand Total'
+
 		invoice.save()
 		invoice.submit()